Determination of residual Ni in mechanically alloyed NiAl
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consolidation.t4] This suggests that alloying continues to take place during milling even though no diffraction peaks for unalloyed material are observed. Furthermore, since the powder was weakly magnetic but the consolidated material was not, it appears that the residual magnetic material in the powder alloys during consolidation. The powder was studied using thermogravimetric analysis (TGA) to determine residual magnetic material by detecting the Curie temperature (Tc). The experimental setup is shown schematically in Figure 1. The powder was heated in the balance of the TGA in the presence of a magnetic field. When a material is heated above its Curie temperature, there is an apparent weight change which is proportional to the amount of magnetic material present. As a reference, TGA for pure Ni powder was carried out by heating up to 600 ~ and cooling to room temperature at the rate of 20 K/min. The result shown in Figure 2 demonstrates the sharpness and extent of the transition for pure material. The Tc determined from this experiment was 358 ~ which is in good agreement with the published Tc for Ni of 354 ~ The initial decrease in the weight on heating is due to desorption of water and other gases such as H. There was a gradual decrease in weight below Tc on cooling, which is due to an increase in magnetic susceptibility. It seems reasonable to suppose that if the residual magnetic material is unalloyed Ni, larger particles should contain more. On the other hand, if it is due to pickup of iron during milling, the smaller particles should contain more. To determine the source of the residual magnetism, powders were sieved, and various size fractions were examined. Figure 3 shows the results obtained on the powder in the range of -450 to +500 mesh. On heating to 600 ~ a distinct Curie temperature is seen at 358 ~ The measured Tc indicates the presence of almost pure Ni in the powder since Tc decreases rapidly with increasing content of A1 in solution in Ni. tT] On cooling, the Curie point is no longer observed, indicating that the residual Ni has alloyed during the brief temperature excursion to 600 ~ The total time of the heating and cooling cycle is 60 minutes, and the diffusion time may be approximated as 3 minutes at 600 ~ using the method of Shewmon. tS] Using the diffusion data of Shankar and Seigle, t91 we can estimate a possible diffusion distance. This results in 16 nm in the case of volume diffusion or 4/xm in the case of boundary diffusion, which is sufficient to homogenize the nanometer scale of the residual Ni. However, the Curie temperature is still observed on cooling coarser powder, but the weight change (AW) is less than that on heating, indicating that residual Ni is only partially alloyed during the temperature cycle. This indicates that either the diffusion distances are larger or the effective diffusion coefficient is smaller in the larger powder. Coarser powders are expected to have larger average grain size than finer powders, and this will reduce the amount of grain boundary transport tha
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